A letter to a dead man lay among the rocks. Pulled from the body of an Islamic State fighter by Afghan troops as they searched corpses sprawled on a rock-strewn hillside in eastern Afghanistan, it issued instructions for distributing captured weaponry.

“Sheik Idriss”, the recipient, was addressed in Urdu, Pakistan’s official language. “When you have weapons from war booty then contact Issa Pakistani,” it read. “He wants to buy those arms including Kalashnikovs. I hope that you will co-operate with him.”

The Afghan troops smiled at the details. The language in which the letter was written, the nationality of Sheikh Idriss and the mysterious Issa all pointed towards their old enemy: Pakistan.

Yet it was the identity of the ­author that stirred their particular interest.

It was signed by Hafiz Saeed Khan, the Pakistani governor of Wilayat Khurasan, Islamic State’s emergent branch in Afghanistan and Pakistan, a man renowned for such wanton cruelty that his terror tactics have shocked even the Taliban. There was no doubting who the Afghan soldiers saw as their greater enemy.

“Hafiz Saeed may receive some sort of internet instruction from Baghdadi in Iraq,” said Brigadier Mohammed Naseem Sangin, referring to the Islamic State leader. He tapped the letter in his hand as sporadic mortar and machinegun fire echoed across the valleys and foothills of a lonely battlefield.

“But this letter, with its instructions from Pakistanis to Pakistanis, backs what we suspect: the Daesh (Islamic State) here in Afghanistan are the project of our neighbour over the border.”

The true origins of Wilayat Khurasan are the subject of contention in Afghanistan, amid a bloody internal struggle within the Taliban leadership.

An increasing body of evidence suggests that Hafiz Saeed Khan and his men are indeed the metastasis of Abu Bakr al-Baghdadi’s organisation in Syria and Iraq, rather than merely being breakaway Taliban opportunists in search of new funding.

“If it looks like a duck, quacks like a duck, it’s a duck,” said a European diplomat in Kabul. “ISIL (Islamic State) are here. We don’t know exactly which way the Daesh phenomena will develop in Afghanistan, but we’ve seen no evidence that they are part of a Pakistani plot. On the contrary, both Afghanistan and Pakistan are at risk from them and should collaborate to tackle this threat.”

Rather than collaborate, Afghan troops on the front line in Nangarhar province, the focus of Islamic State operations, regard them as just the latest creation sent to destabilise their country by Pakistan. “Occasionally you get the odd Afghan with them, a disaffected local Taliban whose group have run out of power or money,” said Brigadier Sangin, the commander of a brigade of Afghan National Army troops whose men are bearing the brunt of the fighting in Abdel Khel. But most of the Daesh we kill here, and 95 per cent of their commanders, are Pakistanis.”

Hafiz Saeed Khan was publicly anointed as Wilayat Khurasan’s leader in January by Muhammad al-Adnani, Abu Bakr al-Baghdadi’s personal spokesman in Syria and Iraq. He was one of six mid-ranking commanders in the Pakistani Taliban, the TTP, to give their oath of allegiance to Baghdadi, forming an Islamic State affiliate that was overwhelmingly drawn from Pakistan’s tribal agencies.

The only Afghan with a senior leadership position in Wilayat Khurasan was Abdul Rauf Khadem, a former Guantanamo Bay inmate originally from the Kajaki district in Helmand. He was reportedly the first to give his oath of loyalty to Baghdadi and is believed to have travelled for consultations with the group in Iraq in October last year. However, his appointment as Hafiz Saeed’s deputy was short-lived — he was killed by a US drone strike in February and replaced with a Pakistani.

Other than its relish for brutality, Wilayat Khurasan is in other respects dissimilar to its parent group in the Middle East.

Brigadier Sangin observed that his men had yet to find a dead foreign fighter — other than Pakistanis — among more than 200 Islamic State fighters they had killed in Achin over the past four months. He claimed that intercepted radio chatter between the militants was in Urdu or Pashtu, never Arabic.

Moreover, Afghan troops fighting around Abdel Khel have not yet experienced a single suicide attack, the hallmark of ISIS tactics in Iraq and Syria.

Whoever his ultimate paymasters might be, Sheikh Idriss never got the chance to obey Hafiz Saeed’s orders on war booty. Shot dead with 15 of his men in a failed night attack on the ridgeline held by Afghan troops in Abdel Khel, Achin district, a fortnight ago, he met the dawn with sightless eyes, leaving Brigadier Sangin and his soldiers to savour their war spoils instead.

“We revenged ourselves well on Hafiz Saeed’s men,” the Brigadier smiled. “Sixteen dead; we took away their weapons and their bodies too.”